Vertical cavity surface emitting lasers (VCSEL) typically include a first distributed Bragg reflector (DBR), also referred to as a mirror stack, formed on top of a substrate by semiconductor manufacturing techniques, an active region formed on top of the first mirror stack, and a second mirror stack formed on top of the active region. The VCSEL is driven by current forced through the active region, typically achieved by providing a first contact on the reverse side of the substrate and a second contact on top of the second mirror stack.
The use of mirror stacks in VCSELs is well established in the art. Typically, mirror stacks are formed of multiple pairs of layers often referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. For example, a GaAs based VCSEL typically uses a GaInP/AlInP, an AlAs/GaAs or an AlGaAs/AlAs material system wherein the different refractive index of each layer of a pair is achieved by altering the aluminum material content in the layers. As an example, in a VCSEL that uses a GaAs substrate and the GaInP/AlInP material system, the emission wavelength is limited to 670 nm or longer. The best results are achieved in the 675-690 nm range, due to poor electrical and optical confinement between the GaInP multiple quantum wells (MQWs) and the AlInP cladding layers used in the active region. In conventional devices of this type, the number of mirror pairs per stack may range from 30-55 to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. The large number of pairs increases the percentage of reflected light.
In conventional VCSELs, conventional material systems perform adequately. However, new products are being developed requiring VCSELs which emit light having visible wavelengths. VCSELs emitting light having a visible wavelength are of great interest in the digital video disk (DVD) industry, more particularly for use with both audio and CD-ROM applications. Accordingly, it would be highly advantageous, to remedy the foregoing problems and other deficiencies inherent in the prior art.
Therefore, it is an object of the present invention to provide a new and improved visible wavelength VCSEL and method of fabrication. Typically, the DVD standard requires an emission in the 635-650 nm range, a power of 3 mW-5 mW for a read only mode of operation, 30 mw for a write only mode of operation, and an operating temperature of up to 60.degree. C. These visible wavelengths can be obtained by using a VCSEL having a strained GaInP quantum well layer and AlGaInP barrier layers which compose the active region, AlInP cladding layers and GaInP/AlInP distributed Bragg reflectors with a GaInP substrate element. In a device of this type, the total bandgap discontinuity is approximately 400 meV, which is large enough to allow for excellent electrical confinement, thus allowing for operation at high temperatures with good reliability. In addition, the distributed Bragg reflectors can be easily formed using alternating layers of GaInP and AlInP layers lattice matched to the GaInP substrate. The refractive index difference between the two layers is approximately 0.5 at 650 nm and a forty pair DBR structure will allow for an approximate 99.99% power reflectance.
Another object of the invention is to provide a reliable visible wavelength VCSEL.
It is yet a further object of the present invention to provide for a new and improved active region for use in a visible wavelength VCSEL.
Another object of the invention is to provide a new and improved substrate element for use in a visible wavelength VCSEL.
Still another object of the present invention is to provide for an efficient active region and mirror stacks for use in a visible wavelength VCSEL.
Yet another object of the invention is to reduce the complexity of fabricating a visible wavelength VCSEL.
Another object of the present invention is to provide an active region which emits visible wavelength light, a mirror stack and substrate which can be lattice matched thereto.